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Student Features

In the winter of 1572, a Danish nobleman admired the chill night sky as he walked home for supper. His attention fell immediately upon a dazzling star, brighter than Venus, where there had been no star before. He was so shocked that he asked his servants to tell him he wasn't dreaming. Then he asked some passing peasants if they too saw the star. For the next eighteen months, the nobleman was obsessed with the new star, or nova, for its appearance would change humanity's view of the universe forever. The nobleman's name was Tycho Brahe, and he was the greatest astronomer of his age.

Image to right: This Chandra image reveals fascinating details of the turbulent debris created by a supernova explosion that was observed by the Danish astronomer Tycho Brahe in the year 1572. The colors show different X-ray energies, with red, green and blue representing low, medium and high energies, respectively. The image is cut off at the bottom because the southernmost region of the remnant fell outside the field of view of the detector. Credit: NASA/CXC/SAO

The sixteenth century was a time of turmoil on Earth, but one thing that was assured was the unchanging nature of the heavens. The seasons changed, the Moon and planets moved against the background of stars, but everything was orderly, predictable. The stars were eternal. They did not come or go. The nova must be earthly, perhaps something high in the atmosphere, just as comets were believed to be. Tycho set about measuring the distance to the nova. He could not tell how far away it was, but he could tell it was further than the Moon, and probably the planets as well. Tycho's nova shattered the myth that the heavens were unchanging with time, and began a revolution in cosmology that is still ongoing.

Tycho's Nova would today be called a supernova, the explosive death of an old star. There are different sorts of supernova, but the one Tycho saw requires a special set of circumstances.

Image to left: An artist's rendering of a supernova in the making. Here one of a pair of stars has become a white dwarf, and is drawing material from its companion, the red star on the left. The white dwarf is very small in comparison to the red star, and is hidden within the white hot disc of hydrogen gas it has acquired. Credit:CXC/M.Weiss

When a star like our Sun runs out of nuclear fuel, it will go through a beautiful death ritual, shedding its outer layers while its inside squeezes down into a dense white-hot ball about the size of the Earth. This ball, aptly named a White Dwarf, is where the story will end for the Sun, but not so for other White dwarfs. This is because most stars live in pairs, a union that becomes dangerous when one of the pair turns into a white dwarf. The white dwarf is greedy - its strong gravity begins to tug at the outer layers of hydrogen gas from its companion and wraps the gas around itself, in the process creating a ticking time bomb. The hydrogen layer grows, getting hotter and hotter until at critical temperature the bomb goes off; a thermonuclear explosion as bright as a billion stars, and a flash that can be seen across the universe.

A valuable property of this type of supernova is that they are all similar sized explosions. So if one supernova looks fainter than another, it must be further away. This property makes them the most accurate tool for measuring the huge distances between galaxies. For when a star in a particular galaxy explodes, we can tell how far the galaxy is by how faint the supernova looks. Measuring distance using this type of supernova gave astronomers the first hint that the expansion of our universe is actually speeding up (see the link below to find out how).

Image above: "Before and After" Images of galaxies taken by the Hubble Space Telescope show very distant supernovae that exploded when the universe was less than half its current age. The apparent brightness of this type of supernova gives cosmologists a way to measure the expansion rate of the universe at different times in the past. Credit: NASA and A. Riess (STScI)

Tycho's observations, made decades before the invention of the telescope, were so good that modern astronomers know his "new" star was a supernova of this type. It would surely make Tycho very happy to see that his discovery in the winter of 1572 is so intimately linked to the cutting edge astrophysics of the 21st century.